1. Field of the Invention
The present invention relates to an optical signal input-type amplifier circuit suitably used for a laser-type bar-code reader that is utilized for POS systems in distribution industry.
2. Description of the Related Art
FIG. 8 is a block diagram showing a structure of a bar code reader. In FIG. 8, numeral 1 represents a bar code printed on a surface of a particle. The bar code 1 is formed of plural black bars and white bars arranged alternately and represents a predetermined data based on the widths of each black bar and each white bar.
The optical system 2 irradiates a laser beam L2 with respect to the bar code 1 and receives the light R1 reflected on the bar code 1. The optical system 2 also is formed of a laser emitting unit 3, a scanning mechanism 4, and a photoelectric converting unit 5.
The laser emitting unit 3 is formed of a semiconductor laser emitting a laser beam L1.
The scanning mechanism 4 is formed of, for example, a polygon mirror driven by a motor. The scanning mechanism 4 also reflects a laser beam L1 emitted from the laser beam emitting unit 3 to irradiate it as a laser beam L2 onto plural black bars and white bars of the bar code 1 while it moves and scans them at a fixed rate and perpendicularly to the black and white bars.
The scanning mechanism 4 also reflects the reflected light R1 of the laser beam L2 reflected on the bar code 1 and irradiates into the photoelectric converting unit 5 the reflection light R1 as a reflection light R2 moving while the laser beam L2 scans it.
The photoelectric converting unit 5 is formed of a photoelectric converting element such as a photo diode. The photo converting unit 5 has a function that converts a reflection light (optical input signal) R2 received via the scanning mechanism 4 into an electric signal (analog signal) corresponding to the light amount thereof and a function as an optical signal input-type amplifier circuit for amplifying the electric signal.
FIG. 9 is a circuit diagram showing a photoelectric converting mechanism 5, or an optical signal input-type amplifier circuit. In FIG. 9, a PIN diode 11 acting as a photoelectric converting element outputs a reverse current corresponding to the intensity of a received light. The PIN diode 11, as shown in FIG. 10(a) to 10(c), has a light receiving portion.
The scanning mechanism 4 needs a wide reading space. As shown in FIGS. 10(a) to 10(c), when the distance between the scanning spot 4a and the condensing lens 30 changes, the light may not be focused on the light receiving surface 11a because the distance between the lens 30 and the receiving surface 11a is fixed. Hence the light receiving surface 11a has a desired area for receiving a large light amount even in the above case.
In order to condense a light reflected at the scanning spot 4a in the scanning mechanism 4 shown in FIG. 8, the condensing means 30 such as a concave mirror or hologram, for example, passes light to focus an image on the light receiving surface 11a of the PIN photo diode 11.
In FIG. 9, numeral 12 represents a resistor with one end grounded, 14 represents a bias voltage applying resistor for applying a bias voltage VG1, and 13 represents a coupling capacitor. Numerals 15 and 18 represent a field effect transistor (FET), respectively. FETs 15 and 18 are connected in parallel to each other. FET 15 has a gate terminal connected to the PIN photo diode 11, and a drain terminal connected to a resistor 16 and a capacitor 17 connected in parallel to each together. FET 18 has a gate terminal connected to the PIN photo diode 11, and a drain terminal connected to a resistor 19 and a capacitor 20 connected on parallel to each other. FET 21 is cascaded to the FETs 15 and 18 with the gate terminals connected to the bias voltage VG2.
Numeral 22 represents a resonance circuit. The resonance circuit 22 is connected as a load to the source terminal of the FET 21 and resonates a predetermined high frequency component containing a frequency corresponding to the bar code information. The resonance circuit 22 is formed of a resistor 22a, a coil 22b, and a capacitor 22c connected in parallel.
The numeral 23 represents a regulator circuit. The regulator circuit 23 drops the voltage of the power source 24 by a desired voltage to make the maximum amplitude of the output signal. The circuit using a Zener diode 27 as shown in FIG. 11 may be used instead of the regulator circuit 23.
That is, the circuit shown in FIG. 11 includes a resistor 26 connected to the power source 24, and a Zener diode 27 and a capacitor 28 connected between one terminal of the resistor 26 and the ground. The resistor 26 produces a predetermined Zener current IZ flowing the Zener diode 27. The capacitor 28 absorbs a noise generated from the Zener diode 27. This circuit can drop by a voltage V1 to maximize the amplitude of the output signal. The drop voltage V1 is set so as to eliminate the difference between the voltage 24V from the power source 24 and the drain/source voltage VDS of FET 21. The drop voltage V1 is expressed by the following formula: EQU V1=24-VDS (1)
Numeral 25 represents a resistor grounded and 29 represents a bipolar transistor for outputting an output signal.
In FIG. 8, numeral 6 represents an A/D converter for converting an electric signal from the photoelectric unit 5 in a digital signal. The A/D converter 6 converts an electric signal into a binary signal including a black level signal corresponding to each black bar portion of the bar code 1 and a white level signal corresponding to each white bar portion of the bar code 1. Generally since the light amount of the light R2 reflected from each white bar portion is larger than that from each black bar portion, the white signal is produced as a high level signal and the black signal is produced as a low level signal.
Numeral 7 represents a bar width counter for counting clock signals from the clock generator 8. The bar width counter 7 produces the time widths of the black level portion and the white level portion of a binary signal from the A/D converter 6, or the values as the counted values of clock signals corresponding to each bar width and each white bar width of an actual bar code.
Furthermore, numeral 9 represents a memory for storing a bar width counted value from a bar width counter 7. The CPU 10 extracts and demodulates predetermined data in the bar code 1 based on the bar width counted value (values corresponding the width of each black bar and each white bar) stored in the memory 9.
In the above structure, the laser beam L1 emitted from the laser emitting unit 3 is irradiated as a laser beam L2 onto the black and white bars of the bar code 1 by the scanning mechanism 4. The laser beam L1 is moved and scanned at a fixed rate while it is scanned perpendicularly to the black and white bars in the bar code 1.
The laser beam L2 emitted from the scanning mechanism 4 is scattered and reflected on a portion of the bar code 1 and then is reentered to the scanning mechanism 4 as a reflection light R1. The reflection light R1 moves in accordance with the laser beam L2 scanning operation while the reflection angle is changing. Since the laser beam L2 is reflected by the polygon mirror forming the scanning mechanism 4, it is inputted as a reflection light R2 to the PIN photo diode 11 being a photoelectric converting element of the photoelectric converting unit 5 arranged at a predetermined position.
The photoelectric converting unit 5 converts reflection light R2 into an electric signal corresponding to the light amount thereof. The electric signal is amplified while the external disturbing light component is removed and it is outputted to the A/D converter 6 as the output of the photoelectric converting unit 5. The A/D converter 6 converts digitally the output into the binary signal including a black level signal correponding to each black bar portion of the bar code 1 and a white level signal correponding to each white bar portion of the bar code 1.
Then the bar width counter 7 counts the clock signals from the clock generator 8 to measure as a counted value of the clock signal the time widths (values corresponding to the width of each black bar and each white bar of an actual bar code 1) of the black and white level signals of the binary signal from the A/D converter 6. The counted value is stored once in the memory 9. The CPU 10 executes a predetermined demodulating process to the bar width counted value stored in the memory 9 to extract and demodulate the predetermined data of the bar code 1.
As described above, in the conventional optical signal input-type amplifier circuit acting as a photoelectric converting unit in a bar code reader, it is considered that a lowered voltage of the power source 24 shown in FIG. 9 or 11 is used to achieve the reduced power consumption and the improved power source working efficiency.
However, it is difficult to realize such a low voltage operation. The reason is that when the voltage of the power source 24 shown in FIGS. 9 and 11 is lowered in use, the drop voltage which occurs by the regulator 23 and the circuit using the Zener diode 27 shown in FIG. 11 must be increased because the drain to source voltage VDS of the transistor 21 is maintained without a change. Hence the difference between power source voltage and the drop voltage occurring in the above circuit must be decreased.
Furthermore, since the bar code reader is required to read at high rate, the scanning mechanism must scan the laser beam at high rate so that the signal inputted to the PIN photo diode is a high frequency signal.
For that reason, the light receiving area of the PIN photo diode must be made large and accordingly the increased parasitic capacitance of the PIN photo diode causes an attenuation of the high frequency component in the amplifier receiving the light signal.
Since the PIN photo diode with a small parasitic capacitance and a large light receiving area is expensive, the boosted manufacturing cost makes difficult to realize an optical signal input-type amplifier circuit driven on a low voltage.
It is possible to reduce the parasitic capacitance of the PIN photo diode by applying a large bias voltage thereon. However, such a countermeasure leads to a complicated circuit, an increased dark current, an increased leakage due to moisture and applied voltage, and an decreased reliability.